Insulator for electric heaters



Sept. 5, Fl yKUHN ET AL INSULATOR FOR ELECTRIC HEATERS Filed June 29, 1936 FIG.I.

FRANK KUHN H Y LAURENCE H.THOMAS /A I'ENTORS Patented Sept. 5, 1939 INSULATOR FOR ELECTRIC HEATERS Frank Kuhn, Detroit, and Laurence H. Thomas, assignors to American Electrical Heater Company, Detroit, Mich., a corporation of Michigan Application June 29, 1936, Serial No. 87,993

Birmingham, Mich.,

3 Claims.

The invention relates to electric heaters and more particularly to the heating units used in laundry irons and similarly manually operated instruments. In the present state of the art such heating elements are usually electrically insulated by mica, but other dielectric materials have also been employed, such for instance as magnesium oxide. While these materials have high dielectric properties, particularly when at ordi-- nary temperatures and in a dry state, a certain amount of electrical leakage will` occur at elevated temperatures or where moisture is present. This may be negligible so far as loss of current is concerned, but is highly objectionable in that it may give an electrical shock to the operator. It is the object of the present invention to avoid such leakage and to this end the invention consists in the construction as hereinafter set forth.

In the drawing:

Figure l is a cross section through an electrical heating unit of our improved construction;

Figure 2 is a similar View showing a modied construction.

As above stated, the usual insulating material employed in heating units is mica. This at low temperatures is highly dielectric, but at the temperature of the element when in use is slightly conductive. Other earthy materials, such for instance as magnesium oxide, are highly dielectric even at eleveted temperatures, but where a1- lowed to stand without heat they absorb moisture which will render them slightly conductive. Thus where either of these materials is alone depended upon for insulation, there is liable to be leakage. To overcome this difliculty, We have constructed a heating unit using in combination a material such as mica which will not absorb moisture and which is dielectric at low -temperatures, with a material such as magnesia which is dielectric when dry and without regard to temperature, but which may absorb moisture. These materials are so positioned as to lie parallel to each other when interposed between the conductor and any grounded element so that either one can be depended upon for effective insulation.

As shown in Figure 1, A is a core member formed of a sheet of mica on which is wound or otherwise secured the resistor B forming the heat generating element. C is a layer of magnesia or other highly dielectric earthy material which is placed over the resistor B to completely cover the same, and D is an outer mica sheet adjacent to the layer C. 'Ihe layer C may be placed in position either as a plastic paste which is then thoroughly dried, or by merely using the comminuted material subjected to high pressure.

In the modified construction shown in Figure 2, the magnesia or equivalent material is placed as a layer C' on the side of the mica sheet D which overlays the resistor B on the core member A.

In either case the low temperature insulating material and the high temperature insulating material are interposed in series between the conductor and the grounded element, such as a sheathing El.

In use when the current is rst passed through the resistor the mica will form an effective insulation and before any temperature can be attained at which the mica becomes conductive, the magnesia or equivalent material will be thoroughly dried so as to prevent all leakage.

What we claim as our invention is:

1. An electrical heating unit comprising a hollow sheathing, a resistor within said sheathing and adjacent layers of mica and magnesia. between said resistor and said sheathing, the magnesia being between the resistor and the mica and in contact with said resistor and said mica.

2. An electrical heating unit comprising a resistor, a layer of magnesia contacting said resistor and an overlayer of mica contacting said magnesia.

magnesia.

FRANK KUHN. LAURENCE H. THOMAS. 

